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| The ground reaction force vector (GRFV) that passes upward from the foot and produces movement at each lower extremity joint. We can visualize the GRFV by studying laboratory investigations of normal gait that employ force plates to measure the GRFV's three-dimensional orientation. |
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The GRFV differs from a conventional "gravity line," which is actually a vector extending vertically from the center of gravity of a static body. Instead, the GRFV is a "reflection of the total mass-times-acceleration product of all body segments and therefore represents the total of all net muscle and gravitational forces acting at each instant of time over the stance period" (Winter, 1984, p.51).
| F=mg designates the vector that represents the force of gravity acting on the objectF=ma designates the vector that represents the instantaneous inertial force acting on this object. In this diagram, inertial forces accelerate the body toward downward and to the right. The bold vector (Fr) is the sum of the gravitational and inertial forces. |
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In contact with a stable surface like the ground, the vector Fr gives rise to an equal and opposite vector, a ground reaction force vector. Since the ground reaction force's vector is equal and opposite to that of Fr, it shares the same line of application and will have the same effect on the body and its joints.
The GRFV combines both gravity's effect on the body and the effects of the body's movement and acceleration (change of velocity) in three planes of reference. This makes the GRFV especially suitable for the study of gait, during which the body's various masses undergo complex accelerations.
Sophisticated gait analysis equipment can generate a visible force vector on an oscilloscope screen and superimpose it simultaneously on a photograph of a gait subject. We can use these to understand the GRFV's effect on the body during walking.
A WORD OF CAUTION: The ground reaction force is not the only force acting on joints during gait. The weight and inertia of a moving segment has an effect on the segments distal and proximal to it. Moving the upper leg influences movement in the lower leg. These joint reaction forces can be important. However, joint reaction forces are relatively small in the lower extremity, at least during stance phase. Therefore, clinicians can use the GRFV's position by itself to understand the forces that human muscles must control during gait's stance phase.
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